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I am confused about the I/V Conversion at the output of this DAC (PCM1798)

In the electrical characteristics section, a dc output current (called "Center Current") of -3.5 mA is given, for zero output. In another DAC we have used(AD1955), this negative current specification is known to actually stand for current flowing into the device, so I am interpreting this specification also as current into the device.

The typical application circuit from the datasheet gives the following I/V conversion circuit for the output. typical application circuit

As far as I can see, the output voltage at I_OUTL- is 0V, which means that the "Center Current" of 3.5 mA would have to flow into the device with zero voltage drop, since the DAC is only supplied with 5V to GND.

There is an evaluation board for this particular DAC, and in that circuit a different configuration is used, where the voltage at I_OUTL- is roughly 0.7V, which would allow current flow into the DAC (the part number on the schematic is different, but the document states that it is valid also for the PCM1798) The eval board is called "DEM-DSD1796/PCM1795/PCM1796/PCM1798" by the way.

Now the question: How is it possible to use the I/V conversion shown in the typical application circuit? (image above)

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Usually, the current spec of a output pin is so that positive values denote current flowing out of the pin. This should be clearly defined in the datasheet.

In your case, it's obvious the pin is sinking current, meaning this "center current" is flowing into the pin. Given no other information, this is what you'd expect from the negative value, but it's also clear that is what is what is expected by the example circuit. The opamps are connected as inverting transconductance amplifiers, with current flowing into the D/A pin causing positive output. The voltage of these D/A pins will be held at 0 by the opamp circuits.

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The device specifies that the "center current" is -3.5mA and the minus sign is important because it tells me that there is probably a negative voltage generator built into the device that can draw current from 0V. This means that the subsequent op-amp design (I to V conversion) does not need a negative supply voltage and of course this is very convenient.

I think you need to look at Vcom L and R - it wouldn't surprise me if the capacitor on those pins was used to smooth a negative voltage generated internally.

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  • \$\begingroup\$ But then the IC would have to have a negative voltage source that can sink at least 4x 3.5 mA, right? \$\endgroup\$ – timor Sep 11 '15 at 10:58
  • \$\begingroup\$ Yes and probably it's internal hence my comment about the capacitor on Vcom \$\endgroup\$ – Andy aka Sep 11 '15 at 11:00

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